The oil pressure relief valve consists of an aluminum plunger with a heavy spring behind it.
Access is through the 15/16" domed cap nut beside the dizzy. Once this nut is removed the spring will be visible. The plunger is beneath the spring and will probably be stuck, requiring encouragement to come out. Briefly cranking the engine over will usually free the plunger; in severe cases a thread tap can be screwed in to grip the plunger from the inside to draw it out.
Now replace the new aluminum plunger(12H865) and lightly grind it in as you would valves in a cylinder head and clean all surfaces afterwards. It is always a good idea to replace the copper washer (6K431) under the domed nut.